WATER TABLES: CHARACTERISTICS, HOW THEY ARE FORMED, POLLUTION - ENVIRONMENT - 2025

The water tables are the layers of free water that accumulate in the soil at a certain depth, saturating it. It is equivalent to the water table, water table, water table or water table, and can be the upper layer of an aquifer or it can be the limit of the soil saturation zone.

In the case of aquifer, it refers to free aquifers, that is, those that have a permeable upper layer of soil that allows recharge. Under these conditions, the water in the aquifer is at atmospheric pressure and the level it reaches is called the water table or water table.

Freatic level. Source: Desireesil / Public domain

In a saturated soil, the water table corresponds to the level reached by the saturated soil layer. Likewise, the depth limit at which this saturation layer begins is called the water table.

The water table is formed when rainwater infiltrates the soil and at a certain depth it encounters an impermeable layer. From this point on, the water accumulates reaching a height defined by the amount of infiltrated water and the area covered.

Water tables are essential for life on Earth, as they provide water to plants through their roots. In the same way, the water table is a source of drinking and irrigation water for humans, extracting it through wells.

Sewage seepage and industrial and mining waste are the main causes of contamination of the water table. Like agricultural and livestock activities, due to the use of agrochemicals in excessive quantities.

Characteristics of the water table

The water table can refer to the zone of saturation of the water in the soil or to an aquifer. In this sense, we speak of an aquifer when the amount of free water available allows its use through wells

For the water tables to be established, several layers have to be formed in the soil:

Layer of infiltration

Above the water table is a layer of permeable soil or rock that allows surface water to infiltrate. The permeable properties of this layer depend on the type of soil and the geological structure of the area.

Waterproof jacket

The infiltrating water must encounter an obstacle that prevents it from continuing its way down, which is the presence of an impermeable layer. It stops the descent of the infiltrated water causing its accumulation and can be rock or clay soil.

Saturation layer or zone

Once its descent has stopped, the water begins to accumulate reaching a certain level or height, which establishes the water table or water table. This process can occur by saturating the pores of the soil or by accumulation of free water in open spaces or pores of permeable rocks.

Layer or zone of aeration or vadose

Upon reaching the corresponding height that makes up the mantle or water table, there is a zone devoid of free water above. This zone where the pores are occupied by air is the vadose or aeration zone or layer.

However, in some cases the water table reaches a superficial level, that is, the saturation zone corresponds to the ground level as occurs in swamp areas.

Loading and unloading

On the other hand, associated with the formation of the water table is the process of loading and unloading the water:

The height of the water table is determined by the relationship between the load and the discharge of water. To the extent that the water that feeds the saturation layer by infiltration is greater, the water table will maintain or increase its level.

Recharge of the water table. Source: Surface_water_cycle.svg: Mwtoewsderivative work: Oxilium / CC BY (https://creativecommons.org/licenses/by/3.0)

At the same time, if the loss of water from the saturation layer is greater than the recharge, the water table or water table will decrease.

The water load comes from precipitation, either directly or indirectly from the water of surface bodies such as rivers or lakes. While the discharge is determined by factors such as evaporation, perspiration, springs and human extraction (wells, drains).

How are the water tables formed?

water in the floor

The soil is more or less porous, depending on its texture and structure, the former being the proportion of sand, clay and silt present. The structure has to do with the aggregates or lumps that are formed, their size, consistency, adherence and other parameters.

The permeability of the soil is important, since it determines the infiltration of the water that falls or runs from the surface to the interior of it. Thus, in a sandy soil the permeability is high because the sand particles leave larger spaces between them.

Whereas in a clay soil the permeability will be lower because the clays leave little or no space in the soil. Therefore, the water will go vertically as deep as the permeability of the substrate allows.

The water will reach a greater or lesser depth depending on the characteristics of the soil and the geological structure of the area. Therefore, when encountering a clayey or impermeable rock layer, its descent will cease and will accumulate, generating a layer saturated with water up to a certain level.

Horizontal displacement is the other dimension of the dynamics of water in the soil and depends on the topography of the terrain. On steep soil, the seeping water will move in the direction of it by gravity.

Subsequently, it accumulates from the lowest level or elevation to which it accesses and reaches a higher or lower water table depending on the amount of water and the extension of the substrate it occupies.

This level will depend on the amount of infiltrated water and its horizontal displacement, and determines the water table or water table.

Aquifer

If water infiltrates a very porous substrate, such as sand or limestone and encounters an impermeable layer, an aquifer forms. If the upper layer of this aquifer is permeable, allowing it to recharge directly, it is a free aquifer.

Types of aquifers. Source: Aquifer it.svg: File: Aquifer en.svg: Hans Hillewaert (Lycaon) derivative work: Bramfabderivative work: Ortisa / Public domain

In this type of aquifers, the water is subjected to atmospheric pressure and therefore the level it reaches is determined by this factor. Under these conditions, the level that reaches the water table of the aquifer is called the water table or water table.

Closed or confined aquifers are those where the water is enclosed between impermeable layers, above and below. Therefore, the water is subjected to pressure within the aquifer that is higher than the ambient pressure.

Due to this, the level that the water reaches when opening a well in a closed aquifer is not the water table but the piezometric level. The latter being the level that the water reaches when it is allowed to flow, in this case determined by the pressure of the confined water (hydrostatic pressure).

Use of the water table by humans

The mantle or water table is important as a source of water for both plants and humans. The presence of a water table at an adequate depth determines the success of certain crops and plantations.

At the same time, too high a water table can prevent cultivation because it causes suffocation of the roots. Similarly, the groundwater table in aquifers is a source of drinking and irrigation water, for the extraction of which wells are built.

Contamination of the water tables

Groundwater is subjected to the intrusion of pollutants that alter its quality in terms of potability. In addition, these waters reach aquatic ecosystems or contaminate the plants that absorb them, affecting biodiversity.

Contamination of the water table. Source: 570ajk / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

These pollutants can come from natural sources, for example heavy metals from soil veins. In this way, groundwater can be contaminated with, for example, arsenic or cadmium.

However, most of the contamination of the water tables is caused by humans. Most human activities generate pollutants that in one way or another can contaminate groundwater.

Solid waste or garbage

The mismanagement of both organic and inorganic solid waste is a major cause of pollution. In those garbage dumps where the land is not properly conditioned, leaks can occur and leachates are generated that go to the water table.

A high proportion of solid waste is plastics and electronic devices, which release dioxins, heavy metals and other toxic substances into the environment. For their part, aqueous solutions of organic waste carry pathogenic microorganisms and toxins to the groundwater table.

Black and gray water seepage

A very dangerous source of contamination of the water table is sewage, which carries a high load of fecal coliforms and other microorganisms. The presence of this type of pollutant makes the groundwater undrinkable, causing infectious diseases.

For its part, gray water contributes detergents, fats and various polluting substances to the groundwater.

Agricultural activity

Agricultural and livestock activities are a source of contamination of the water table, especially due to the use of agrochemicals. Herbicides, insecticides, and fertilizers add nitrates, phosphates, and other toxic substances to the water.

This occurs when applied to the soil and crops, being washed by irrigation or rain water, filtering up to the water table. Similarly, feces and sewage from animal farms contaminate the waters of the groundwater table.

Runoff waters

Rainwater carries away all kinds of waste substances as it runs superficially through agricultural lands, industrial estates and urban areas. This contaminated water ends up seeping into the ground and reaching the groundwater table.

Industrial and mining spills

Solid and liquid wastes from industries are a major source of highly hazardous pollutants. This includes heavy metals, acids, industrial detergents, lubricants, and other substances.

For its part, mining generates highly toxic waste that reaches the groundwater, polluting it. In the case of gold mining, the use of arsenic, cyanide, mercury and other dangerous substances is an example of this.

Similarly, the extraction and transport of oil are a source of contamination of the water table with heavy metals, benzene and other toxic derivatives.

Acid rain

It drags nitric and sulfuric acids from the atmosphere that help to release heavy metals from the soil that are dragged to the water table. In the same way, they acidify surface and groundwater.

References

1. Calow P (Ed.) (1998). The encyclopedia of ecology and environmental management.
2. Custodio, E., Llamas, MR and Sahuquillo, A. (2000). Challenges of underground hydrology. Water engineering.
3. Gupta A (2016). Water pollution-sources, effects and control. https://www.researchgate.net/publication/321289637_WATER_POLLUTION SOURCESEFFECTS_AND_CONTROL
4. Ordoñez-Gálvez, JJ (2011). Groundwater - Aquifers.. Technical primer. Geographical Society of Lima.
5. Sahuquillo-Herráiz, A. (2009). The importance of groundwater. Rev. R. Acad. Science. Exact. Fis. Nat. (Esp.).
6. Viessman Jr, W. And Lewis, GL (2003). Introduction to Hydrology. Pearson.
7. Wyatt CJ, Fimbres, C., Romo, L., Méndez, RO and Grijalva, M. (1998). Incidence of Heavy Metal Contamination in Water Supplies in Northern Mexico. Environmental Research.